Buds and Budding
A bud is a swelling on a plant stem, consisting of numerous overlapping, immature (i.e., not-yet developed), primordial leaves or floral structures (such as petals). The growth of plant buds occurs through the action of a region of rapidly dividing and differentiating tissue, known as an apical meristem.
A plant bud is a complex, compound structure, whose detailed anatomy is revealed by careful dissection and examination using a microscope. The smallest, least developed, leaf or floral primordia occur in the center of a bud, while the older, larger, more developed ones occur on the outside and curve up and over the younger ones, enveloping them in a sheath-like manner. In the center, beneath the youngest primordia, is the meristematic tissue. The dormant buds of many woody plants are protected by several tough, overlapping structures known as bud scales. When used in reference to botanical structures of this type (i.e., plant buds), the term "budding" describes the developmental process by which buds expand due to tissue growth and differentiation, and eventually develop into mature leaves or floral structures.
The buds of many plants have episodic growth, meaning they are dormant for long periods of time, only growing when environmental circumstances are favorable. In climates that are strongly seasonal, either because of cold winters or extended drought, buds are stimulated to grow (or "break") when the temperature warms in the springtime, or after rains occur. In such cases, the dormancy of buds is broken by the occurrence of specific concentrations of certain plant hormones, known as auxins (the amounts of the auxins are regulated by biochemical mechanisms).
In the stems of many plants, a dilute stream of auxins moves down the shoot from the bud at its apex, and this serves to inhibit the growth of buds lower on the stem (this is known as apical dominance). However, if the terminal bud on the shoot is injured (for example, by being browsed by a deer, or trimmed by a gardener), this stream is interrupted. This biochemical change results in previously dormant, lateral buds lower on the shoot breaking their auxin-enforced dormancy. This allows new, lateral shoots to develop and grow upwards, to replace the terminal bud-shoot that was previously growing (and dominant).
An alternative definition of "budding" refers to a kind of non-sexual reproduction, in which a new "individual" develops as a direct growth from a body tissue of the parent. The new individual may become detached from its parent, to grow as a discrete organism. However, the new individual is genetically identical to its parent (unlike the case in sexual reproduction). Yeasts are single-celled fungi that reproduce by budding new cells from the parent ones. In plants, asexual reproduction by budding can occur in various ways, such as the development of runners (i.e., above-ground stems, such as those of the strawberry), rhizomes (underground stems that grow outward from the parent, such as those of poplars), plantlets developed at the edges of leaves (as occurs in species of mother plant, or Kalanchoe), or bulbils produced in leaf axils (such as those of the tiger lily). Some animals also reproduce asexually by budding, as is the case of many species of hydra, corals, and sea anemones.
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